The statements in this section merely provide background information related to the present disclosure and do not constitute prior art.
A video image is formed of a series of pictures, and each picture is divided into predetermined regions such as blocks. When the region of an image is divided into blocks, the divided blocks are classified into an intra block and an inter block depending on an encoding scheme. The intra-block means a block that is encoded based on an intra-prediction coding scheme. The intra-prediction coding scheme predicts pixels of a current block by using pixels of neighboring blocks that have been already encoded and decoded within a current picture, so as to generate a predicted block, and encodes pixel differences between the predicted block and the current block. The inter-block means a block that is encoded based on an inter-prediction coding scheme. The inter-prediction encoding scheme predicts a current block in a current picture by referring to at least one previous picture and/or at least one subsequent picture, so as to generate a predicted block, and encodes the difference between the predicted block and the current block. Here, a picture that is referred to in encoding or decoding the current picture (i.e., current frame) is called a reference picture.
Image signal coding is performed to reduce the image size (volume). In other words, an image compression is performed. The image compression takes the prediction in two ways, that is, an intra prediction and an inter prediction. The prediction serves to generate predicted image signal similar to an original signal and transmit a difference between the original signal and an anticipated to the original signal. Assuming pixel values are 100, 200, 150, 240 and 178 with their binary numbers expressed as 100=1100100(2), 200=11001000(2), 150=10010110(2), 240=11110000(2) and 178=10110010(2), there are 40 bits used in total. However, when these values are predicted into, e.g. 95, 190, 149, 240 and 178 and encoding is performed only on a residual signal that is the difference between the predicted signal and the original signal, the encoded residual signal is expressed as 5=101(2), 10=1010(2), 1=1(2) and 0=0(2) which need substantially reduced bits to be used for encoding and in turn reduced size of the overall image to be transmitted to a decoding apparatus. The inventor(s) has noted that the better prediction achieves the higher coding efficiency.